1. Field of the Invention
The present invention relates to a technique for operating a photovoltaic power generator including solar cells and a storage battery mounted on a house or the like and, more particularly, to a technique for operating a plurality of photovoltaic power generators capable of reverse power flow of electric power generated by the solar cells back to the electric power system.
2. Description of the Background Art
From the viewpoint of conserving the global environment, use of various energies has been reviewed. In particular, solar cells that utilize solar energy are considered promising as a representative source of clean energy. Recently, systems have been developed in which dispersed power sources on the side of solar cells and the commercial power supply on the side of the electric power system are linked, so that if necessary power consumed by a house cannot be fully provided by the solar cells, the shortage is offset by the commercial power supply on the side of the electric power system. Among such systems, some are adapted to realize reverse power flow, to feed excessive power output by solar cells back to the electric power system.
Further, a technique for storing excessive power output by solar cells in a storage battery and using the electric power of the storage battery if necessary power consumed by a house cannot be fully provided by the solar cells has been known. By way of example, Japanese Patent Laying-Open No. 2003-79054 discloses a solar power generation system having a storage battery. According to Japanese Patent Laying-Open No. 2003-79054, a solar power generation system, linked to an electric power system, supplies the power generated at a solar cell device to an inverter device for conversion to AC power and feeds it to a power consuming unit, and the system includes a storage battery for storing power from the solar cell device, and changeover control means for switching output of the power from the solar cell device to the storage battery or to the inverter device. The solar power generation system controls charging of the storage battery such that the battery is charged by at least one selected from the power generated by the solar cell device at a off-peak time of electric demand after sunrise and power from the electric power system at night, and controls discharging of the power stored in the storage battery in accordance with the change curve of electric demand, with the demand high in a specific time zone, so that the power from the battery is applied to the inverter device in addition to the power generated by the solar cell device.
When a plurality of photovoltaic power generators having storage batteries are connected to one receiving end of the electric power system, however, the following problem arises. Specifically, each photovoltaic power generator starts charging immediately after sunrise. Therefore, power is hardly sold through the receiving end of the electric power system in the morning. When the storage battery begins to be fully charged (FULL) in the afternoon based on the relationship between the amount of power generation and the amount of stored electric power of the solar cell, there may be a case where each photovoltaic power generator starts selling power almost concurrently through the receiving end of the electric power system.
Therefore, the amount of power sold through the receiving end of the electric power system in the morning is extremely small, while the amount of power sold through the receiving end of the electric power system in the afternoon becomes very large. In other words, the amount of electric power sold at the receiving end of the electric power system fluctuates considerably, so that the state of operation of the power operation system including a plurality of photovoltaic power generators connected to the receiving end of the electric power system becomes unstable. This leads to difficulties in maintaining the quality of the electric power, that is, suppressing voltage fluctuations and frequency fluctuations, for electric power companies managing the electric power system.
Even in the case where the electric power around the receiving end frequently becomes unstable, the electric power companies are responsible for maintaining the quality of the electric power, which causes a problem of an increase in the cost of the facility for maintaining the quality. Consequently, it is expected that electric power companies show reluctance to purchase the electric power. There is also a probability that the electric power companies may refuse to purchase the electric power in the areas and the circumstances where the widespread use of the solar cells is concentrated.